Lasers are widely known for their use in industrial applications for cutting through steel, or for their potential as defensive weapons; they bring to mind powerful, visible, searing beams. It's less typical for lasers to connote cooling. But a team of researchers at Technische Universität Dortmund (Dortmund, Germany) and Ruhr-Universität Bochum (Valbonne, France) have used the "reverse" effect of laser cooling to cool semiconductor material.1
In this process, known as photoluminescence up-conversion, the frequency of the laser beam is carefully chosen so that the irradiated material does not absorb light, but rather relinquishes some of its energy to the beam. The photons in the laser beam absorb some of the oscillation energy of the particles in the material (its "phonons"), reducing the temperature of the target (see www.laserfocusworld.com/articles/325783). While this cooling effect has been examined extensively in its application to atomic vapor and condensed matter, such as dye solutions and glass, its use with semiconductor materials had not been established.
In a paper published in Physical Review B, Soheyla Eshlaghi and colleagues revealed their discovery that the anti-Stokes photoluminescence (PL) used to cool the target is temperature dependent. The cooling efficiency of a laser increases as the temperature of the sample increases. Therefore, control of the sample temperature in concert with the laser wavelength will determine the optimal cooling efficiency.
The group also determined that the thickness of the gallium-arsenide layer, usually a few dozen atoms thick, can be designed to determine the amount of cooling. Because the quantum well structure can be created with the precision of one atomic layer, the cooling technology may someday have application in semiconductor-lasers. Not only could vibration-free cooling of semiconductors become practical, the laser diode generating the light may someday be used to effectively cool its own structure.
The study was carried out at the department of experimental physics, Technische Universität Dortmund, by Dr. Soheyla Eshlaghi, Wieland Worthoff, and Professor Dieter Suter, as well as professor of applied solid-state physics Andreas D. Wieck at Ruhr-Universität Bochum.
REFERENCE
1. S. Eshlaghi et al., Physical Review B 77, 245317 (2008).
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